Propeller



H. L. MILNER Jan. 13, 1948.

PROPELLER 2 Sheets-Sheet 1 Filed March 25, 1944 x Q a a a 4 5. a J 9.5. m p? a a a 0 lfnvanibr 73 Z. Jfzlinar H. L. MlLNER Jan. 13, 1948.

PROPELLER Filed March 25, 1944 2 Sheets-Sheet 2 m w 5 EW m m O LWM W M M 4W Fatented Jan. 13, 1948 PROPELLER Harry Lawley Milner, Cheltenhain, England,as' signor to Rotol Limited, Gloucester, England,

a British company Application March 25, 19451, Serial No, In Great Britain March 1-9, 1943 Section 1, Public Law 690, August 8;;1943 Patent expires March 1-9, 1963 1 Claim. 1

In the manufacture of apropeller or airscrew, great care has hitherto been taken to ensure that it is statically balanced to such a degree of accuracy that when in use it shall not give rise to objectionable or dangerous vibration. Since, however, it is impossible to make each blade perfect, and manufacturing tolerances have to be allowed, differences are found to exist between different blades.

In the specification of my United States patent application No. 464,086, filed October 31, 1942, I have described a method of manufacturing screw-propellers whereby a satisfactory degree of aerodynamic balance can be achieved, and any tendency of the propeller to give rise to vibration arising from a lack of such balance was reduced. Described in general terms, that method consisted in adjusting the pitch-settings f the blades of a propeller by a predetermined small amount so as to produce" the required result. In carrying out that method certain calculations were involved and for practical purposes at that date, some approximations could be made and some factors could be neglected whilst still obtaining results that were efiective. Thus, for example, it was permissible to neglect variations in the torque-forces between different blades arising from fortuitous differences in their shape and dimensions, one reason being that I found it was satisfactory to correct any differences in the thrust-moments of different blades and that such corrections nearly always gave at least a partial correction for the variations in torque-forces which accompanied them. It was also permissible in making the calculations aforesaid to neglect the forward speed of the aircraft, takingaccount only of the rotational speed of the propeller.

The enhanced performance of aircraft now available as regards speed, and the increased power now absorbed by aprope-ller have rendered of appreciable importance the two factors which were previously negligible, and the object of the present invention is to provide a more improved method of'manufacturi'ng. propellers.

This invention accordingly comprises a method of determining the angle at which a propellerblade should be set with reference to the basic pitch angle in order that the torque-force of theblade may approximate so closely tothe datum' torque-force as to provide a satisfactory degree of aerodynamic balance-when the blade is assembled in a propeller at its appropriate angle with another blade or other blades each set at its appropriate angle, Which'method consists in (a) 2 measuring by physical means the variations form and dimensions of the blade from t ose" of a datum blade,- (11) e'stimati'ngfrom such measure ment the difference in torqu'e force of the blade: from the datum ttrdue-forceyane (c) calculating therefrom the said first mention'ed angle.

The datum blade above referred to may be a hypothetical bl'a'de deemed to have been manu-' factured exactly in accordance with the design drawings, or maybe an actualma'ster blade made to correspond as" closely possible to s'u'c'h draw in'g's, or it may be a" suitable normal blade arbitrarily adopted as a standard of comparison;

The datum torqu 'force of any blade is the tolciue foi ce ofa" datum blade which has been set to the basic pi-tchang-le}and the'basic pitch angle is the'angle'at W hiCIia? datum bladeshouldbe Set whri beihg assemble-dun the propeller to suit the particular cCli'fditions" under Whiohit is'rquile'd to operate. v V I v,

The step (b)" of the method above set forth, namely assessing the differences in torque for'ce of the" blade from those of a" datum blade, ay be" effected in accordance with the simple blade element tneoryusiiig the samefapproxiriiat'ionsas} described in my prior speciiication aforesaid, or a more accurate assessment of the torque-force can be made by themethodhereinafter set forth in detail. I

According td anetherr amrje' of this invention, stepfa) aforesaid'corisists in" determining at each" of a; numberof stations along' thefblade the? Variation'in the" angular position of the zero-lift line from that of a datum blade, as for example I by'means of the gauge described in the specification of my United- States patent application No; 4643087, which'issued'on June 25; 1946, as Patent No. 2,402,567 inthe'same manner as de'scribed'inf my United States patent application No. 464,086;

A'ccordingto another feature 'of this invention, a method of determining theahgle airscrew bladeshould be setin'order to: providea satisfactory degree of; aerodynamic balance when the-blade iaa'ssembled' in an airscrewcon-" sists in (a) measuring by physical means the variations inform andv dimensions of the blade from thoseofa datum'blade, (b)- estimating from such measurements the difference both-in torqueforce-and-thrust-momentof the blade'from those of a datum blade, (0) calculating therefrom the angle to which the blade should be set to'correct either of them, and (d) when=siichangles 'o'f cor' rection differ, selecting either ofthem or a' suit able intermediate value, appropriate to the op erating conditions of the propeller. The said at which an 3 selection will usually be made empirically since it will depend to some extent upon factors which cannot be predetermined, such as the structural design of the machine in which the propeller is installed.

According to yet another feature of the invention, it comprises a method of making a propeller which consists in manufacturing the blades within suitable tolerances, estimating the angle at which each blade should be set with reference to the basic pitch-angle by any of the methods above set forth, and assembling each blade at its appropriate angle so that the propeller shall have a satisfactory degree of aerodynamic balance.

When an assessment of the difference in thrust-moment from that of a datum blade is made, the method used may be that set forth in my prior specification aforesaid, or may be the more accurate method hereinafter set forth in which account is taken of the forward speed of the machine. The estimation of the torque-force and of the thrust-moment, and the calculation of the angle by which the pitch-setting of the blade should be adjusted will now be explained in detail and for this purpose reference Will be made to the diagrams Figures 1 to 7 of the drawings accompanying this specification.

In these drawings:

Figure 1 represents the cross section of a blade element and the components of velocity,

Figure 2 represents the corresponding components of force of the blade,

Figures 3, 4 and 5 illustrate the processes used in the present invention in the evaluation of am,

Figure 6 illustrates a two-bladed airscrew on one blade of which air forces exert an excess torque, and Figure 7 illustrates diagrammatically a three-bladed airscrew in accordance with the present invention.

Consider in the first place the effect of thrust distribution. If T is the total thrust of a blade and r is the radius of the center of thrust, a condition for smooth running of an airscrew is that the moment of thrust should be the same for all the component blades, or

Tr=constant In the simple blade element theory of airscrews the blades are regarded as aerofoils under the influenceof a resultant effective air velocity which changes in magnitude and direction along the blade.

Consider the forces acting on a blade element of radius r and length 0:7 Figure 1 represents the cross section of a blade element and the components of velocity. Figure 2 represents the corresponding components of force of the blade.

The symbols in these diagrams have the following significance:

n =angular velocity of the airscrew V=forward speed of flight W=resultant air velocity relative to the blade element a =angle between the zero-lift line of the section and direction of W (angle of incidence) 0 =angle of zero-lift line relative to the plan of rotation dL=lift force on the element dD=Drag force on the element C =chord of the section The forces on the element are related to the quantities represented in Figure 1 by the follow ing equations:

where Cr, and CD are lift and drag coefliclents of the blade section and p is the air density.

The axial thrust dT of the element is given by the components of these forces parallel to the axis of rotation.

Similarly the torque dQ due to the element is the moment of the component forces about the axis of rotation.

Now CD is a small quantity compared with CL and, over the range of angles of incidence associated with an efficient airscrew, is substantially constant.

Also, C'z.=ma where m is some constant.

Hence, neglecting terms containing C1) and substituting for CL, the moment of the thrust component about a plane containing the axis of rotation and perpendicular to the blade axis is dM=TdT=21r 1L pmCoLT sec. 41611 (3) and similarly dQ=21r TL pmCaT tan. sec. Mr (4) If R denotes the tip-radius of the airscrew and by introducing Equations 3 and 4 may be written where K1 and K2 are constants for the blade.

The change in thrust-moment due to a change 60: in the angle of incidence extending over a blade-segment bounded by radii 1, and .52 is given by Imagine the blade divided into segments bounded by sections at radii 1, 2, 3 1 in which the angular errors of the zero-lift line are 5011, 50:2, 6113, etc., the excess of thrust-moment over that of the datum blade will be, with due regard to sign,

But the same change in thrust-moment could be brought about by rotating the datum blade through some angle Aer say. Conversely the 7-. ing the observed sections in the prescribed manner:

(1) The observed sections being of substantially equal importance, errors of observation will tend to cancel out whereas if the usual drawing sections are employed the weight-factors become very unequal and put an undue responsibility on one or two measured sections.

(2) By adopting sections which are not specifled in the manufacturer's drawing and in consequence are not subject to close inspection or control, there is greater probability of discovering aerodynamic discrepancies than would otherwise be the case.

So far, the method of weighting the blade section errors has been founded on approximate airscrew theory which, however, despite the approximations adopted and because the final results are obtained as a quotient of two quantities containing similar approximations, yields results of sufficient accuracy for most practical applications. There is no difliculty in attaining even greater accuracy should this be desired. A convenient process in this case would be to plot a thrust grading curve for the blade in accordance with more elaborate airscrew theory for the desired operating conditions (this corresponds to Figure 5) and then construct a moment grading curve by multiplying each ordinate of the first curve by its radius to obtain a curve corresponding to Figure 4 and applying the same process to the new curves as was employed with Figures 4 and 5.

For perfect aerodynamic balance of an airscrew the total thrust-moment of the several blades should be zero and the torque-forces about the axis of rotation should be equal.

A method of obtaining thrust-moment balance has already been dealt with and it now remains to examine the effects of unequal blade-torques. Forthis purpose a two-bladed airscrew will be considered as this exemplifies the process for any number of blades. Let the air forces on one blade exert an excess torque of value Q0 which may be represented by a force P acting at a radius T, Figure 6.

This will result in an unbalanced force P acting laterally on the propeller shaft and rotating with it. For smooth running we should have P=0.

Taking the tip-radius of the blade as unity, the torque exerted by a blade-element at radius a: and of length do: is given by Equation 6 above, that is to say:

dQ Kal /x ids (6) Applying a similar process to that used in discussing the thrust-moment and adopting the same nomenclature, the angular change in bladeangle required to make P=0 is given by or for any'combination of the two according to the relative importance of the disturbing influences. Fortunately, correction for unbalanced thrust will, with certain unimportant exceptions, also reduce the corresponding unbalanced torque and vice versa.

The variation with radius of the torque and thrust weight-factors We and. WT of a typical blade for two values of J and are given in the following table.

Section radius We W I' An examination of the above table shows that the relative importance of aerodynamic errors at the respective blade sections changes with the value of J, or in other words according to the forward speed of the aircraft, and that the root and tip sections are affected more than the intermediate sections.

It will also be noted that the weight-factors for torqueand thrust-moment, whilst of the same order at intermediate radii, for the same value of J are appreciably difierent at root and tip.

It will further be clear that only a fortuitous disposition of aerodynamic errors along a given blade will result in a simultaneous elimination of both thrust and torque effects and in general one may expect only to be able to eliminate one of these effects with a consequent reduction in the value of the other.

Consider an uncorrected two bladed propeller in which one blade exerts more thrust and torque than the other. The thrust-moment will give rise to a couple having its axis in the plane of rotation and perpendicular to the blade axis. The resultant torque of the two blades will give rise to a force acting in the direction of the axis of the couple and both the couple and force rotate with the propeller thus giving rise to a resultant periodic bending moment about any plane in the airframe parallel to the propeller disc. The magnitude of this bending moment obviously depends upon the distance between the plane and the propeller disc. The response to the disturbing resultant will depend upon the dynamic characteristics of the aeroplane and may be more pronounced in one aircraft than in another. Similar effects will be observed in a propeller having more than two blades.

These vibrational disturbances will be minimised if not entirely eliminated by correcting the propeller blades in accordance with the foregoing formulae for either thrust-moment or torque or some optimum combination of the two having regard to the particular aircraft to which the propeller is fitted.

The airscrew illustrated in Figure '7 comprises a hub I0 having a central opening II by which it is mounted at its driving shaft, and is provided with three sockets I2, I3, I4 which receive respectively blades I5, I6, IT. The root ends of the blades are cylindrical and are adjustable in their respective sockets. On each socket there is provided a datum mark I8, and the root end of each blade has marked on it an index mark I9, 20, 2I respectively. If a blade is found to have the desired aerodynamical characteristics, its index mark is set in alignment with the datum mark on the socket as shown in the case of the blade [5. If the herein described method of assessing the aerodynamical characteristics of the blade shows that a correction must be applied to this setting, the index mark is displaced from the datum by the desired amount. The blade 16 is shown as adjusted in one direction, and the blade I? is shown as adjusted in the opposite directionfor positive and negative corrections. It will be appreciated that the pitch angles at which the blades are thus set would not necessarily be diiierent, since each is derived as the result of distinct measurement and computation.

It will be appreciated that the foregoing analysis and the method of manufacturing propellers, whilst described in relation to airscrews and blades thereof, is equally applicable to marine screw-propellers, the only difierence bein the substitution of hydrodynamic forces and effects for aerodynamic forces and efiects.

I claim:

An airscrew comprising blades each assessed as to its aerodynamic qualities by the steps of (a) measuring by a convenient empirical method the Id direction relatively to a fixed datum of the zerolift line of the section of a master blade at a number of stations along the blade, (b) measuring by the same empirical method any diflerences in direction of the corresponding lines of the blade being assessed, (0) determining the torque force produced by such differences, and (d) calculating the departure from the zero-lift setting of the master blade which would produce an equal torque force, and a hub to which said blades are assembled and adjusted to diiferent pitch angles such as will compensate for said differences in torque force.

HARRY LAWLEY MILNER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,693,450 McCauley Nov. 27, 1928 1,769,767 Caldwell July 1, 1936 2,219,303 Fraser Oct. 29, 1940 2,336,303 Schubert Dec. 7, 1943 2,343,383 Martin Mar. 7, 1944 

